1. Field of the Invention
The present invention relates to an object detecting apparatus configured to detect a target by transmitting an electromagnetic wave, and by receiving a reflected wave resulting from reflection of the electromagnetic wave off the target.
2. Description of the Related Art
When a radar beam transmitted from a radar system is refracted due to a prism effect of ice crystals adhering to a radome of the radar system, and a ghost beam is accordingly outputted in a direction different from a direction of a main beam, the radar system may undesirably detect a ghost target and the actual or true target by receiving the ghost beam and the reflected wave of the main beam off the true target. Thus, the radar system may erroneously detect a ghost target that does not exist.
To address this issue, a radar system which avoids the erroneous detection of a ghost target is disclosed in Japanese Patent No. 3676625. The radar system deletes ghost data based on the fact that the distance to the true target and the distance to the ghost target are equal to each other; and that the receiving level of the reflected wave of the ghost target is lower than the receiving level of the reflected wave of the true target.
Several reasons are attributed to the detection of the ghost target by the radar system.
One reason is that, as shown in FIG. 5A, the radar system outputs side lobe beams in directions which are offset to the left and right at a predetermined angle (approximately 4 degrees in general) from the direction of the main beam, respectively. Although each side lobe beam is weaker than the main beam, the radar system detects a ghost target when receiving a reflected wave of the side lobe beam.
Another reason is that, as shown in FIG. 5B, the radar system outputs a ghost beam in a direction which is offset to the left or right at a predetermined angle from the direction of the main beam because ice crystals adhering to the surface of the radome, or flaws or the like in the surface of the radome, act like a prism on the main beam. Although the ghost beam is weaker than the main beam, the radar system detects a ghost target when receiving a reflected wave of the ghost beam.
Yet another reason is that, as shown in FIG. 5C, the traveling direction of the main beam changes when being reflected by a roadside object, such as a guard rail. The radar system detects a ghost target when receiving the reflected wave of such a main beam after the reflected wave is again reflected by a target.
Referring to FIGS. 6A to 6D, detailed descriptions will be provided for the reason why a ghost target is detected when a side lobe beam is outputted.
Suppose a case where, as shown in FIG. 6A, a preceding vehicle, which serves as a detection target, runs in an adjacent left lane of a lane in which a subject vehicle runs, and where a main beam directed to the left at an angle of 4 degrees from the forward direction of the vehicle body detects the preceding vehicle. In this case, a side lobe beam directed to the left at an angle of 4 degrees from the direction of the main beam is not reflected by the preceding vehicle. For this reason, a ghost target is not detected.
When the main beam is scanned to the right to be directed ahead of the vehicle body, as shown in FIG. 6B, the main beam is not reflected by the preceding vehicle, but the side lobe beam is instead reflected by the preceding vehicle. In this case, a ghost target of the preceding vehicle is detected in the direction of the main beam (ahead of the vehicle body of the subject vehicle).
As shown in FIG. 6C, a data set for one cycle composed of the two foregoing cases includes a real data set on the adjacent left lane; and a ghost data set on the lane in which the subject vehicle runs. For this reason, the radar system erroneously recognizes that two preceding vehicles arc running side by side at the same speed in the two respective lanes at the same distance from the subject vehicle. Due to such erroneous detection, an unnecessary decelerating control operation may be carried out to avoid getting closer to the ghost target on the subject vehicle's lane, which may confuse the driver.
As shown in FIG. 6D, the invention described in Japanese Patent No. 3676625 solves the foregoing problem by deleting, as ghost data, one of two data sets that indicate that their targets are detected at the same distance and have a certain difference between their receiving levels. The deleted data set has a lower receiving level than the retained data set.
However, the radar system disclosed in Japanese Patent No. 3676625 may cause a problem under a specific condition, which will be described by referring to FIGS. 7A to 7C.
Suppose a situation where, as shown in FIG. 7A, a truck and a passenger vehicle run side by side, respectively, in the left adjacent lane and the lane in which the subject vehicle runs, at the same distance from the subject vehicle and at the same speed, and where a main beam directed to the left at an angle of 4 degrees relative to the subject vehicle detects the truck.
As shown in FIG. 7B, when the main beam is scanned to the right to be directed ahead of the vehicle body of the subject vehicle, the passenger vehicle running in the subject vehicle's lane is detected.
As shown in FIG. 7C, a data set for one cycle composed of the two foregoing cases includes a data set which indicates the truck running in the adjacent left lane has a higher receiving level; and a data set which indicates the passenger vehicle running in the subject vehicle's lane has a lower receiving level. In addition, the two data sets indicate that their respective detection distances are equal to each other. For this reason, the condition is similar to that shown in FIG. 6D, and accordingly the data set of the passenger vehicle, which is real or actual data, may be erroneously deleted for being ghost data.